J. Chem. Eng. Data 1998, 43, 299-303
299
Phase Equilibria in the Systems Methyl 1,1-Dimethylethyl Ether + Benzene and + Toluene Ricardo Reich,*,† Marcela Cartes,† Jaime Wisniak,*,‡ and Hugo Segura† Department of Chemical Engineering, Universidad de Concepcio´n, Concepcio´n, Chile, and Department of Chemical Engineering, Ben-Gurion University of the Negev, Beer-Sheva, Israel 84105
Pure-component vapor pressures and vapor-liquid equilibrium data at 94 kPa have been determined for the binary systems methyl 1,1-dimethylethyl ether + benzene and methyl 1,1-dimethylethyl ether + toluene. The systems exhibit regular behavior and a slight positive deviation from ideality, and no azeotrope is present. The data were correlated by the Wisniak-Tamir equation, and the appropriate parameters are reported.
The Reformulated Gasoline Program of the United States requires that gasoline must fulfill strict requirements on ozone-forming and air toxic emissions. These goals are achieved today by addition of oxygenates such as methyl 1,1-dimethylethyl ether (MTBE), ethyl 1,1dimethylethyl ether (ETBE), methanol, and ethanol. MTBE is the primary oxygenated compound used around the world. Although aromatics such as benzene and toluene may be present in small concentrations in a typical gasoline, they represent fundamental examples of mixtures of MTBE with an aromatic compound. Vapor-liquid equilibrium data for the system MTBE + benzene have been reported at (323.17, 343.15, and 363.05) K by Jin et al. (1985) and at 313.15 K by Lozano et. (1997). For the system MTBE + toluene, vapor-liquid equilibrium data have been reported at 363.15 K by Plura et al. (1979). The isothermal data published in the literature suggest that both systems present small positive deviations from ideality, behave essentially as regular solutions, and present no azeotrope. The present work was undertaken to measure vapor-liquid equilibria (VLE) data for the title systems for which isobaric data are not available. It is also part of our experimental program to determine vaporliquid equilibrium data for binary and ternary systems composed of an oxygenate and one or more gasoline components. Experimental Section Purity of Materials. Methyl 1,1-dimethylethyl ether (99.95 mass %), benzene (99.9+ mass %), and toluene (99.80 mass %) were purchased from Aldrich. The reagents were used without further purification after gas chromatography failed to show any significant impurities. The properties and purity (as determined by GLC) of the pure components appear in Table 1. Appropiate precautions were taken when handling MTBE, in order to avoid peroxide formation, and benzene, a human carcinogen. Apparatus and Procedure. An all-glass vapor-liquidequilibrium apparatus model 601, manufactured by Fischer Labor und Verfahrenstechnik (Germany), was used in the * Corresponding authors. E.mail: bgumail.bgu.ac.il. † Universidad de Concepcio ´ n. ‡ Ben-Gurion University of the Negev.
[email protected];
wisniak@
Table 1. Mole Percent GLC Purities (mass %), Refractive Index nD at the Na D Line, and Normal Boiling Points T of Pure Components component (purity/mass %)
nD(293.15 K)
T/K
methyl 1,1-dimethylethyl ether (99.95)
1.369 22a
327.85a
benzene (99.96)
1.369 0b 1.500 72a
328.35b 353.18a
toluene (99.80)
1.501 11c 1.496 88a
353.21c 383.65a
1.496 94c
383.76c
a
Measured. b TRC Tables, a-6040. c TRC Tables, a-3200.
equilibrium determinations. In this circulation-method apparatus, the solution is heated to its boiling point by a 250-W immersion heater (Cottrell pump). The vaporliquid mixture flows through an extended contact line that guarantees an intense phase exchange and then enters a separation chamber whose construction prevents an entrainment of liquid particles into the vapor phase. The separated gas and liquid phases are condensed and returned to a mixing chamber, where they are stirred by a magnetic stirrer, and returned again to the immersion heater. The temperature in the VLE still has been determined with a Systemteknik S1224 digital temperature meter and a Pt100 Ω probe calibrated at the Swedish Statens Provningsanstalt with the IPTS-68 temperature scale. The accuracy is estimated as (0.02 K. The total pressure of the system is controlled by a vacuum pump capable to work under vacuum up to 0.25 kPa. The pressure has been measured with a Fischer pressure transducer calibrated against an absolute mercury-in-glass manometer (22-mm diameter precision tubing with cathetometer reading); the overall accuracy is estimated as (0.02 kPa. On the average the system reaches equilibrium conditions after 1-2 h of operation. Samples, taken by syringing 1.0 µL after the system had achieved equilibrium, were analyzed by gas chromatography on a Varian 3400 apparatus provided with a thermal conductivity detector and a Tsp model SP4400 electronic integrator. The column was 3 m long and 0.3 cm in diameter, packed with SE-30. Column, injector, and detector temperatures were (343.15, 413.15, 493.15) K for the system MTBE + benzene and
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300 Journal of Chemical and Engineering Data, Vol. 43, No. 3, 1998 Table 2. Experimental Vapor-Liquid Equilibrium Data for Methyl 1,1-Dimethylethyl Ether (1) + Benzene (2) at 94 kPa
T/K
x1
y1
γ1
350.77 350.07 349.24 348.56 347.98 347.15 346.80 345.76 344.96 343.82 342.64 340.91 339.35 337.15 335.43 333.40 332.07 330.54 328.98 327.67 326.80 326.25 326.06 325.60
0.000 0.015 0.034 0.051 0.064 0.085 0.093 0.121 0.146 0.177 0.210 0.267 0.323 0.412 0.480 0.574 0.632 0.714 0.802 0.885 0.944 0.978 0.992 1.000
0.000 0.037 0.079 0.115 0.142 0.181 0.197 0.246 0.285 0.332 0.387 0.462 0.529 0.614 0.670 0.744 0.786 0.847 0.900 0.943 0.973 0.989 0.996 1.000
1.199 1.174 1.149 1.148 1.122 1.123 1.112 1.098 1.085 1.103 1.088 1.074 1.043 1.029 1.014 1.014 1.015 1.008 0.997 0.992 0.991 0.988 1.000
γ2
-B11/ cm3 mol-1
-B22/ cm3 mol-1
-B12/ cm3 mol-1
1.000 0.999 0.998 0.997 0.998 1.001 1.001 1.002 1.001 1.007 1.001 1.000 0.999 1.014 1.037 1.057 1.067 1.035 1.029 1.060 1.058 1.084 1.251
1005 1011 1016 1021 1027 1029 1037 1043 1052 1062 1076 1088 1107 1122 1139 1151 1165 1180 1192 1201 1206 1208
988 994 999 1003 1010 1012 1020 1026 1035 1044 1058 1070 1089 1103 1121 1133 1147 1162 1174 1183 1188 1190
985 991 995 1000 1006 1008 1016 1022 1031 1040 1053 1066 1084 1098 1116 1127 1141 1155 1168 1176 1181 1183
(373.15, 423.15, 543.15) K for the system MTBE + toluene. Very good separation was achieved under these conditions, and calibration analyses were carried out with synthetic mixtures to convert the peak ratio to the mass composition of the sample. The pertinent polynomial fits had a correlation coefficient R2 better than 0.99 and a mole fraction standard deviation of 4 × 10-4 for the system MTBE + benzene and of 6 × 10-4 for the system MTBE + toluene. At least three analyses were made of each composition. Concentration measurements were accurate to better than (0.001 mole fraction.
Table 3. Experimental Vapor-Liquid Equilibrium Data for Methyl 1,1-Dimethylethyl Ether (1) + Toluene (3) at 94 kPa
T/K
x1
y1
γ1
381.04 380.47 378.74 377.24 375.24 373.00 370.62 369.13 367.07 364.86 362.89 360.71 358.93 357.55 355.77 353.12 350.83 348.83 345.66 342.41 340.53 337.28 335.18 332.81 330.53 329.21 327.79 327.06 326.37 326.05 325.60
0.000 0.004 0.017 0.029 0.046 0.064 0.085 0.098 0.118 0.141 0.163 0.187 0.209 0.225 0.252 0.291 0.326 0.363 0.428 0.489 0.530 0.611 0.670 0.749 0.811 0.866 0.919 0.954 0.979 0.990 1.000
0.000 0.021 0.083 0.132 0.193 0.260 0.328 0.366 0.420 0.474 0.516 0.557 0.595 0.621 0.653 0.696 0.731 0.767 0.809 0.849 0.873 0.906 0.928 0.949 0.967 0.976 0.985 0.990 0.993 0.995 1.000
1.214 1.165 1.137 1.117 1.128 1.125 1.133 1.130 1.127 1.113 1.107 1.102 1.108 1.090 1.077 1.072 1.063 1.037 1.044 1.046 1.034 1.028 1.011 1.020 1.005 0.999 0.990 0.990 0.991 1.000
γ3
-B11/ cm3 mol-1
-B33/ cm3 mol-1
-B13/ cm3 mol-1
1.000 0.999 0.996 0.996 0.998 0.996 0.994 0.994 0.990 0.988 0.992 1.000 0.996 0.994 0.999 1.009 1.016 0.998 1.020 1.012 0.988 0.997 0.980 0.986 0.938 0.999 1.099 1.369 1.974 2.951
816 826 834 845 857 871 880 892 906 918 932 944 953 965 983 1000 1014 1038 1063 1079 1106 1124 1145 1165 1178 1191 1198 1205 1208
1221 1236 1249 1267 1287 1310 1324 1344 1366 1387 1410 1429 1444 1465 1496 1523 1548 1589 1632 1658 1705 1737 1774 1810 1832 1856 1869 1881 1886
998 1010 1020 1034 1050 1068 1079 1095 1112 1128 1146 1161 1173 1189 1213 1234 1253 1284 1317 1337 1373 1397 1425 1452 1469 1486 1496 1505 1509
Results The temperature T and liquid-phase xi, and vapor-phase yi mole fraction measurements at P ) 94 kPa are reported in Tables 2 and 3 and Figures 1-4, together with the activity coefficients γi, which were calculated from the following equation (Van Ness and Abbott, 1982)
ln γi ) ln
( ) Pyi
P0i xi
+
(Bii - VLi )(P - P0i ) + RT P 2RT
∑∑y y (2δ i k
ji
- δjk) (1)
where T and P are the boiling point and the total pressure, VLi is the molar liquid volume of component i, P0i is the pure-component vapor pressure, Bii and Bjj are the second virial coefficients of the pure gases, Bij is the cross second virial coefficient, and
δij ) 2Bij - Bjj - Bii
(2)
The standard state for calculation of activity coefficients is the pure component at the pressure and temperature of the solution. The pure-component vapor pressures P0i were determined experimentally as a function of the temperature, using the same equipment as that for obtaining the VLE data; the pertinent results appear in Table 4.
Figure 1. Boiling-point diagram for the system methyl 1,1dimethylethyl ether (1) + benzene (2) at 94 kPa: experimental data (b); smoothed curve obtained from the regular model (s).
The measured vapor pressures were correlated using the Antoine equation
log(P0i /kPa) ) Ai -
Bi (T/K) - Ci
(3)
where the Antoine constants Ai, Bi, and Ci are reported in Table 5. The vapor pressures were correlated with an average percentual deviation in pressure (APDP) of 0.07% for methyl 1,1-dimethylethyl ether and 0.03% for toluene and benzene. The parameters presented in Table 5 predict
Journal of Chemical and Engineering Data, Vol. 43, No. 3, 1998 301
Figure 2. Activity-coefficient plot for the system methyl 1,1dimethylethyl ether (1) + benzene (2) at 94 kPa: γ1 calculated from experimental data (O); γ2 calculated from experimental data (b); smoothed curve obtained from the regular model, eq 4 (s).
Figure 4. Activity-coefficient plot for the system methyl 1,1dimethylethyl ether (1) + toluene (3) at 94 kPa: γ1 calculated from experimental data (O); γ2 calculated from experimental data (b); smoothed curve obtained from the regular model (s). Table 4. Experimental Vapor Pressures Determined for the Pure Species MTBE
Figure 3. Boiling-point diagram for the system methyl 1,1dimethylethyl ether (1) + toluene (3) at 94 kPa: experimental data (b); smoothed curve obtained from the regular model (s).
very well the experimental vapor pressures measured by Ambrose (1981) for benzene [APDP ) 0.27%] and by Myers et al. (1979) for toluene [APDP ) 0.49%], and by Wu et al. (1991) for MTBE [APDP ) 0.53%]; however, larger differences were found with the data of Ambrose et al. (1976) for MTBE [APDP ) 1.03%]. A graphical comparison of the vapor pressure data measured in this work and other references is presented in Figure 5. The molar virial coefficients Bii and Bjj were estimated by the method of Hayden and O’Connell (1975), using the molecular parameters suggested by Prausnitz et al. (1980) and assuming the association and solvation parameters to be negligible. Critical properties of both components were taken from DIPPR (Daubert and Danner, 1989), and liquid volumes were estimated from the Rackett relation (Smith and Van Ness, 1987), which uses only critical properties of the compound. The last two terms in eq 1 contributed less than 6% to the activity coefficients in both binaries, and their influence was important only at very dilute concentrations.
benzene
toluene
T/K
P/kPa
T/K
P/kPa
T/K
P/kPa
299.11 301.17 302.47 305.42 307.16 308.68 311.10 312.99 314.33 315.98 317.20 319.07 320.95 323.13 324.37 325.75 327.85
35.01 38.02 40.06 45.04 48.19 51.07 56.04 60.16 63.27 67.21 70.22 75.08 80.19 86.54 90.22 94.49 101.33
295.78 305.91 311.70 316.69 320.44 323.21 325.82 329.32 332.49 335.41 337.18 339.37 341.72 343.83 345.77 348.34 350.93 353.18
11.42 18.03 23.04 28.16 32.65 36.31 40.04 45.56 51.07 56.62 60.16 64.85 70.18 75.28 80.15 87.06 94.49 101.33
320.32 330.37 336.36 341.04 346.38 351.49 354.05 356.47 359.18 361.93 365.41 367.98 370.20 372.05 373.97 376.49 379.09 380.92 383.65
10.94 16.63 21.08 25.16 30.63 36.71 40.12 43.56 47.69 52.21 58.42 63.35 67.89 71.88 76.20 82.21 88.82 93.71 101.33
Table 5. Antoine Coefficients, Eq 3a compound
Ai
Bi
Ci
methyl 1,1-dimethylethyl ether benzene toluene
5.860 78 6.088 17 6.223 72
1032.988 1243.256 1432.925
59.880 48.640 43.929
a Obtained from the correlation of experimental vapor-pressure data.
The calculated activity coefficients are reported in Tables 2 and 3 and are estimated to be accurate to within 3%. As seen from Tables 2 and 3 and Figures 2 and 4, the binary systems formed by methyl 1,1-dimethylethyl ether (1) + benzene (2), and + toluene (3) behave as essentially ideal, with activity coefficients slightly greater than unity. The thermodynamic consistency of the systems was checked using the point-to-point test proposed by Van Ness et al. (1973), as modified by Fredenslund et al. (1977); for both systems the consistency criteria is met with a oneparameter Legendre polynomial, or regular model, which reduces the functionality of the excess Gibbs energy GE to
302 Journal of Chemical and Engineering Data, Vol. 43, No. 3, 1998
Figure 5. Comparison of correlated vapor pressures with other references: experimental data of Ambrose et al. (1976) for 1,1dimethylethyl ether (3); experimental data of Ambrose (1981) for benzene (O); experimental data of Myers et al. (1979) for toluene (0). Predicted by eq 3 and parameters in Table 5 for methyl 1,1dimethylethyl ether (s), for benzene (‚‚‚), and for toluene (-‚‚-).
Figure 6. Consistency residuals for the system methyl 1,1dimethylethyl ether (1) + benzene (2) at 94 kPa: pressure residuals, P/ kPa ) Pcalcd - Pexptl (b); vapor-phase composition residuals, y ) ycalcd - yexptl (O).
Table 6. Consistency Results MAD(P)/ 100 kPab MAD(y)a
binary system methyl 1,1-dimethylethyl ether (1) + benzene (2) methyl 1,1-dimethylethyl ether (1) + toluene (3)
Ac
0.31
0.33
0.122
0.36
0.37
0.139
a MAD(y): mean average deviation in vapor-phase composition residuals, y ) ycalcd - yexptl. b MAD(P): mean average deviation in pressure, P ) Pcalcd - Pexptl. c A: parameter in eq 4.
the following symmetric relation
GE ) Ax1x2 RT
(4)
for which consistency statistics, and the A parameter, are given in Table 6. In addition, consistency residuals are reasonably distributed, as can be seen in Figures 6 and 7. Following the model of consistency (eq 4), it can be established that the systems measured in this work behave like regular solutions, in excellent agreement with the results of Plura et al. (1979) for the system methyl 1,1dimethylethyl ether (1) + toluene (2) at 333.15 K, for which a value of A ) 0.137 was found in eq 4. Although eq 4 provides satisfactory thermodynamic consistency according to the test criteria, as shown by Figures 6 and 7, the correlation does not fit the data of experimental activity coefficients too well, as depicted by Figures 2 and 4. Higher-order terms in the Legendre polynomial were rejected because although the consistency criteria was satisfied, the polynomial wriggle yielded an absurd dependence of activity coefficients in composition. The scattering of the experimental activity coefficients in Figures 2 and 4 may be explained in terms of the order of magnitude of the limiting activity coefficients (γ∞i ) that, being close to the unity, are largely affected by experimental errors, which, in turn, are magnified by the narrow scale of the figures. It should be noted that in common nonideal
Figure 7. Consistency residuals for the system methyl 1,1dimethylethyl ether (1) + toluene (3) at 94 kPa: pressure residuals, P/kPa ) Pcalcd - Pexptl (b). Vapor-phase composition residuals, y ) ycalcd - yexptl (O). Table 7. Coefficients in Correlation of Boiling Points, Eq 6, for the Systems Methyl 1,1-Dimethylethyl Ether (1) + Benzene (2) and Methyl 1,1- Dimethylethyl Ether (1) + Toluene (3) at 94 kPa, Average Deviation, and Root-Mean-Square Deviations in Temperature, rmsd system
C0
C1
C2
max dev/Ka
avg dev/Kb
rmsd/Kc
1+2 1+3
-13.45 -46.53
9.14 28.68
2.98 -3.85
0.51 0.92
0.16 0.24
0.05 0.06
a Maximum deviation deviation.
b
Average deviation. c Root-mean-square
systems (where γ∞i > 2) the activity coefficient plot shows a wider scale, where absolute errors similar to those obtained in this work are masked by the order of magnitude of i. The boiling-point temperatures of the solution at 94 kPa were correlated with its composition by the equation
Journal of Chemical and Engineering Data, Vol. 43, No. 3, 1998 303 proposed by Wisniak and Tamir (1976) m
T/K ) x1T01 + x2T02 + x1x2
∑C (x k
1
- x2)k
(5)
k)1
In this equation T0i /K is the boiling point of the pure component i at the operating pressure and m is the number of terms in the series expansion of (xi - xj). The various constants of eq 5 are reported in Table 7, which also contains information indicating the degree of goodness of the correlation. Literature Cited Ambrose, D.; Ellender, J. H.; Sprake, C. H. S.; Townsend, R. Thermodynamic Properties of Organic Oxygen Compounds. XLIII. Vapor Pressures of Some Ethers. J. Chem. Thermodyn. 1976, 8, 165-178. Ambrose, D. Reference Values of Vapour Pressure. The Vapour Pressure of Benzene and Hexafluorobenzene. J. Chem. Thermodyn. 1981, 13, 1161-1167. Daubert, T. E.; Danner, R. P. Physical and Thermodynamic Properties of Pure Chemicals. Data Compilation; Taylor & Francis: Bristol, PA, 1989. Fredenslund, A.; Gmehling, J.; Rasmussen, P. Vapor-Liquid Equilibria Using UNIFAC; Elsevier: Amsterdam, 1977. Hayden, J.; O’Connell, J. A Generalized Method for Predicting Second Virial Coefficients. Ind. Eng. Chem. Process Des. Dev. 1975, 14, 209216. Jin, Z.-L.; Lin, H.-M.; Greenkorn, R. A.; Chao, K.-C. Vapor-Liquid Equilibrium in Binary Mixtures of Methyl tert-Butyl Methyl Ether + n-Hexane and + Benzene. AIChE Symp. Ser. 1985, 81 (244), 155160. Lozano, L. M.; Montero, E. A.; Martin, M. C.; Villaman˜a´n, M. A. Excess Thermodynamic Functions for Ternary Systems Containing Fuel Oxygenates and Substituted Hydrocarbons. 1. Total-Pressure Data
and Ge for Methyl tert-Butyl Ether-Benzene-Cyclohexane at 313.15 K. Fluid Phase Equilibr. 1997, 13, 163-172. Myers, J. E.; Hershey, H. C.; Kay, W. B. Isothermal P, x, y Relations, Activity Coefficients, and Excess Gibbs Free Energies for Heptamethyltrisiloxane + Toluene at 343.15, 363.15 and 383.15 K. J. Chem. Thermodyn. 1979, 11, 1019-1028. Plura, J.; Matous, J.; Nova´k, J. P.; Sobr, J. Vapour Liquid Equilibrium in the Methyl tert-Butyl Ether-n-Hexane and Methyl tert-Butyl Ether-Toluene Systems. Collect. Czech. Chem. Commun. 1979, 44, 3627-3631. Prausnitz, J. M.; Anderson, T.; Grens, E.; Eckert, C.; Hsieh, R.; O’Connell, J. Computer Calculations for Multicomponent VaporLiquid and Liquid-Liquid Equilibria; Prentice Hall: Englewood Cliffs, NJ, 1980. Smith, J. M.; Van Ness, H. C. Introduction to Chemical Engineering Thermodynamics; McGraw-Hill Book Co.: New York, 1987; p 97. TRCsThermodynamic TablessHydrocarbons; Thermodynamics Research Center: The Texas A&M University System, College Station, TX, extant 1996; a-3200, 1994. TRCsThermodynamic Tables-Non-Hydrocarbons; Thermodynamics Research Center: The Texas A&M University System, College Station, TX, extant 1996; a-6040, 1963. Van Ness, H. C.; Byer, S. M.; Gibbs, R. E. Vapor-Liquid Equilibrium: Part I. An Appraisal of Data Reduction Methods. AIChE J. 1973, 19, 238-244. Van Ness, H. C.; Abbott, M. M. Classical Thermodynamics of Nonelectrolyte Solutions; McGraw-Hill Book Co.: New York, 1982. Wisniak, J.; Tamir, A. Correlation of the Boiling Point of Mixtures. Chem. Eng. Sci. 1976, 31, 631-635. Wu, H. S.; Pividal, K. A.; Sandler, S. I. Vapor-Liquid Equilibria of Hydrocarbons and Fuel Oxygenates. J. Chem. Eng. Data 1991, 36, 415-421.
Received for review October 16, 1997. Accepted January 20, 1998. This work was financed by FONDECYT, Chile, Project No. 1960583.
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